BI-2536

Polo-like kinase 1 (PLK1) and bromodomain 4 (BRD4) are well-known oncoproteins that drive tumor cell growth in many cancer types. Simultaneously targeting these protein targets has been intently pursued by scientists to enhance anti-cancer effect in chemotherapy. However, it is rare to design proteolytic targeting chimeras (PROTAC) to degrade these oncoproteins simultaneously by one single molecule. Herein, we designed and synthesized seven PROTAC molecules based on BI-2536, a dual-target inhibitor of BRD4 and PLK1. Among these, compound 17b demonstrated the best ability to degrade PLK1, BRD4 and other BET family proteins. The dual targeting PROTAC 17b induces the almost complete degradation of BET proteins and PLK1 at concentration as low as 3 nM, but proteolysis of PLK1 takes place a lot later than BET proteins (36 h vs 4 h). Compound 17b exhibited strong anti-proliferative activities across multiple cancer cell lines. Furthermore, 17b was able to regulate the expression of downstream genes involved in key cellular processes and exert the prolonged suppression of cancer cell growth. These findings suggest that 17b is a highly potent and efficacious dual-targeting degrader.

Polo-like kinase 1 (Plk1) is an important cell cycle regulator that is a recognized target for development of anti-cancer therapeutics. Plk1 is composed of a catalytic kinase domain (KD), a flexible interdomain linker and a polo-box domain (PBD). Intramolecular protein-protein interactions (PPIs) between the PBD and KD result in "auto-inhibition" that is an essential component of proper Plk1 function. Recently, we developed high-affinity PBD-binding inhibitors using a bivalent approach. These ligands contain the low-nanomolar affinity Plk1 KD-binding inhibitors BI2536 or Wortmannin tethered to the PBD-binding peptide, PLH*SpT (H* represents a -(CH₂)₈Ph group on the histidine side chain π-nitrogen). Due to the extremely high affinity of these bivalent inhibitors, to avoid bottoming out in competitive binding assays, it was necessary to use PLH*SpT in the affinity probe. As reported herein, we have developed fluorescence polarization assays using a new fluorescent probe based on the Plk1 PBD-binding peptide, FDPPLHSpTA. We applied the assay to evaluate the affinities of bivalent inhibitors that possess a variety of PBD-binding peptides having much lower PBD-affinities than PLH*SpT. Tethering BI2536 in these bivalent inhibitors resulted in significant affinity enhancements as compared to the parent monovalent peptides.